Ceramic mirrors

ABSTRACT

A ceramic mirror having a surface flatness of no more than 1 μm and/or a centerline average height of no more than 1 nm as measured by the laser interference method, is made from a titanium oxide powder having an average particle diameter of 0.5 μm and a minimum purity of 99.8% by weight. The mirror also has a maximum amount of irregular reflection of incident light not exceeding 5% when the angle of projection is between 0° and 90° as measured by a colorimeter and is made by the process including forming a titanium oxide powder having a maximum average particle diameter of 0.5 μm and a minimum purity of 99.8 wt. %, compacting the titanium oxide powder by cold isostatic pressing to produce a compact, sintering the compact of titanium oxide powder in air at a temperature of 1000° C.-1300° C. to produce a ceramic body, hot isostatic pressing the ceramic body after the sintering in an inert atmosphere at 800° C.-1500° C. under a minimum pressure of 500 kg/cm 2 , and machining the sintered body by surface grinding and lapping.

This is a divisional application of application Ser. No. 08/283,666 filed Aug. 1, 1994 U.S. Pat. No. 5,492,872 which in turn is a continuation application under 37 CFR §1.62 of prior application Ser. No. 08/064,044, filed May 20, 1993, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to ceramic mirrors for use in laser and X-ray apparatuses, cameras, and other precision optical instruments and the like.

Mirrors for use with precision optical instruments and the like are predominantly made of glass, ruby, sapphire and the like, and also of single crystal ceramics. In recent years alumina and SiC have partly come in use as polycrystalline ceramics.

Glass-based mirrors are made relatively thick to make up for the inadequate strength and fragility, and therefore the heavy weight and large size of the mirrors as a whole have been a problem. Polycrystalline alumina, SiC, and other ceramic materials are used in the form of sintered bodies after grinding and lapping. None of them have proved fully satisfactory in respect of the properties such as surface smoothness and optical properties, because the pores inherent to the ceramic materials remain exposed on the lapped surfaces.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a mirror having excellent surface smoothness with fewer pores than heretofore known and having superior optical properties due to lower irregular reflection of light.

The present inventor, in an effort to solve these problems, sintered a high-purity, fine titanium oxide powder at a relatively low temperature. This has been found to give a mirror having excellent surface smoothness with a smaller number of pores than before and having superior optical properties due to less irregular reflection of light. The discovery has now led to the perfection of the present invention.

The invention resides, in essence, in a ceramic mirror characterized by a surface flatness of no more than 1 μm and/or a centerline average height of no more than 1 nm as measured by the laser interference method. The method of manufacturing a ceramic mirror of the invention is characterized by the steps of compacting a titanium oxide powder having an average particle diameter of no more than 1 μm and a purity of at least 99% and sintering the compact in an ordinary, inert, vacuum, or reducing atmosphere at a temperature between 1000° and 1300° C.

DETAILED DESCRIPTION

The invention will now be described in detail.

A fine titanium oxide powder of high purity is used as a material and is sintered at a lower temperature than the usual sintering temperatures to obtain a sintered body with excellent surface smoothness, inhibiting the particle growth and preventing the pore size increase while achieving an adequate sintered density.

The reason why a ceramic mirror having outstanding characteristics can be obtained by the aforesaid method is theoretically to be clarified yet. Possibly the low temperature sintering of a fine titanium oxide powder keeps the pores within the crystal grains (a phenomenon known as "ghosting"), suppressing the accumulation and growth to a grain boundary phase.

The expression "a lower temperature than the usual sintering temperatures" as used herein means a temperature range from 1000° to 1300° C. If the temperature is below 1000° C. the sintering itself does not proceed. A temperature above 1300° C., on the other hand, causes rapid grain growth and therefore a decrease in mechanical strength and pore migration and growth to a grain boundary phase. Consequent reduction of surface smoothness and irregular reflection of light renders it impossible to obtain a mirror of satisfactory optical characteristics.

If the material powder is not at least 99% pure titanium oxide or has a larger average particle diameter than 1 μm, it is no longer easily sinterable at low temperatures. Sintering in the temperature range of 1000° to 1300° C. will not give a product with an adequate sintered density.

For this reason it is desirable that the titanium oxide powder have a purity of at least 99%, preferably at least 99.8%, and an average particle diameter of no more than 1 μm, preferably no more than 0.5 μm.

The titanium oxide powder is compacted by CIP (cold isostatic pressing), mechanical pressing, doctor blade method, extrusion, or other technique depending on the configuration of the product. Whatever compacting technique is chosen, an organic binder or the like suited for the particular method is used as a compacting assistant. The compact thus obtained is sintered in air, vacuum, or an atmosphere of N₂, Ar, H₂ or the like, at sintering temperatures in the optimum range of 1000° to 1300° C. Where greater surface smoothness is required, the compact is hot press sintered under a pressure of at least 50 kg/cm². Such a sintered body is further treated by hot isostatic pressing (HIP) under a pressure of not less than 500 kg/cm².

The TiO₂ sintered body so obtained is machined, e.g., by surface grinding or lapping, to a mirror for use in a laser or X-ray apparatus, camera, or other precision optical instrument. When necessary, the mirror is coated with a metal of high reflectivity so as to be a mirror for a precision optical application.

Such a ceramic mirror has a surface flatness of no more than 1 μm and/or a centerline average height of no more than 1 nm. Moreover, the amount of its irregular reflection of incident light measured with a colorimeter is 5% or less. If the characteristic values exceed the numerical values specified above, too much irregular reflection will result, making the optical characteristics inadequate for a mirror.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples 1 to 4 of the present invention and Comparative Examples 5 to 8 are given below.

EXAMPLE 1

To a titanium oxide powder having an average particle diameter of 0.25 μm and a purity of 99.8% ("CR-EL" (trade name), a product of Ishihara Sangyo Kaisha) was added 2% polyvinyl alcohol (PVA) as a compacting assistant. After mixing in a mortar and drying, the mixture was sieved for particle size regulation.

The mixture was compacted by CIP at a pressure not lower than 1000 kg/cm² and sintered at ordinary pressure in air at 1200° C. The sintered body so obtained was-lapped, and its surface flatness and amount of irregular reflection were measured. The results are given in Table 1.

    ______________________________________                                                                                Amount                                         TiO.sub.2 powder   Sur-   Center-                                                                              of ir-                                                  Aver.           face line  regular                             Sam-     Pu-    part.           flat-                                                                               aver. reflec-                             ple      rity   dia.   Sintering                                                                               ness height                                                                               tion                                No.      (%)    (μm)                                                                               condition                                                                               (μm)                                                                             (nm)  (%)                                 ______________________________________                                         Exam- (1)    99.8   0.25 Sintered at                                                                             0.9  0.8   4                                 ple                      ordinary                                                                       pressure in air                                                                at 1200° C.                                          (2)    "      "    Sintered at                                                                             0.8  0.7   3                                                          ordinary                                                                       pressure in Ar                                                                 at 1100° C.                                          (3)    "      "    Hot press                                                                               0.4  0.5   2                                                          sintered in N.sub.2                                                            at 1000° C.                                          (4)    "      "    After sintering                                                                         0.2  0.3   1                                                          in air at                                                                      1200° C.,                                                               HIP'd in Ar at                                                                 1000° C. and                                                            1800 kg/cm.sup.2                                      Comp. (5)    96.0   0.25 Sintered at                                                                             1.8  3.2   10                                Ex                       ordinary                                                                       pressure in air                                                                at 1200° C.                                          (6)    "      "    Sample 5 1.4  2.7   8                                                          sintered in air                                                                at 1000° C.                                                             and HIP'd at                                                                   1800 kg/cm.sup.2                                            (7)    99.5   1.5  Sintered at                                                                             2.4  6.5   15                                                         ordinary                                                                       pressure in air                                                                at 1200° C.                                          (8)    "      "    Sample 7 sin-                                                                           2.0  4.1   13                                                         tered at 1000°                                                          C. and HIP'd                                                                   at 1800 kg/cm.sup.2                                   ______________________________________                                    

EXAMPLE 2

The titanium oxide powder of Example 1 was used. Following compacting, it was sintered in an Ar atmosphere at 1100° C. The properties of the sintered body after lapping are shown at (2) in Table 1.

EXAMPLE 3

Using the titanium oxide powder of Example 1, hot press sintering was performed in an N₂ atmosphere at 1000° C. and at a pressure of 200 kg/cm². The sintered body after lapping exhibited properties as shown in Table 1.

EXAMPLE 4

The sintered body obtained in Example 1 was treated by HIP in an Ar atmosphere at 1000° C. and at a pressure of 1800 kg/cm².

Table 1 gives the properties of the sintered body as lapped.

COMPARATIVE EXAMPLE 5

Sample No. 5 was obtained by following the same procedure as described in Example 1 with the exception that a titanium oxide powder having an average particle diameter of 0.25 μm and a purity of 96.0% was employed instead. The properties of the sintered body after lapping are shown in Table 1.

COMPARATIVE EXAMPLE 6

Sample No. 5 was compacted and sintered in air at 1000° C. and at ordinary pressure and then treated by HIP under the conditions given in Example 4 to obtain Sample No. 6. The sintered body upon grinding showed properties as in Table 1.

COMPARATIVE EXAMPLE 7

Except for the use of a titanium oxide powder having an average particle diameter of 1.5 μm and a purity of 99.5%, the procedure of Example 1 was repeated, when Sample No. 7 was obtained. The sintered body properties after grinding are given in Table 1.

COMPARATIVE EXAMPLE 8

Sample No. 7 was sintered in air at 1000° C. and at ordinary pressure and treated by HIP under the conditions of Example 4 to obtain Sample No. 8. Table 1 shows the sintered body properties after grinding.

Industrial Applicability

As described above, sintering a high-purity, minute titanium oxide powder at a low temperature below the usual sintering temperature range makes ghosting of pores in the resulting sintered body possible. In this way titanium oxide sintered parts have been obtained which exhibit high enough surface smoothness and optical characteristics for mirrors to be used in laser and X-ray apparatuses, cameras, and other precision optical instruments. 

What is claimed is:
 1. A titanium oxide mirror comprising:a ceramic body having a mirror surface with a maximum centerline average height of 1 nm as measured by a laser interference method, said ceramic body made from a compacted titanium oxide powder having an average particle diameter no greater than 0.5μm and a minimum purity of 99.8 wt %.
 2. A titanium oxide mirror as claimed in claim 1 wherein:said mirror surface having a maximum value of irregular reflection of incident light of 5% as measured with a colorimeter with an angle of projection between 0° and 90°.
 3. A titanium oxide mirror comprising:a ceramic body having a mirror surface with a maximum centerline average height of 1 nm as measured by a laser interference method and a maximum value of irregular reflection of incident light of 5% as measured with a colorimeter with an angle of projection between 0° and 90°, made by the process comprising: forming a titanium oxide powder having a maximum average particle diameter of 0.5 μm and a minimum purity of 99.8 wt. %; compacting said titanium oxide powder by cold isostatic pressing to produce a compact; sintering said compact of titanium oxide powder in air at a temperature of 1000° C.-1300° C. to produce a sintered body; hot isostatic pressing said sintered body in an inert atmosphere at 800° C.-1500° C. under a minimum pressure of 500 kg/cm² to produce a ceramic body; and machining said ceramic body by grinding and lapping its surface.
 4. A titanium oxide mirror, as claimed in claim 3, wherein:said average particle diameter of titanium oxide powder is 0.25 μm.
 5. A titanium oxide mirror, as claimed in claim 3, wherein:said sintering temperature is 1200° C., and said hot isostatic pressing is in argon at a temperature of 1000° C. under a pressure of 1800 kg/cm².
 6. A titanium oxide mirror, as claimed in claim 3, wherein:said irregular reflection of said mirror is 1%; and said centerline average height is 0.3 nm.
 7. A titanium oxide mirror, as claimed in claim 4, wherein:said sintering temperature is 1200° C., and said hot isostatic pressing is in argon at a temperature of 1000° C. under a pressure of 1800 kg/cm².
 8. A titanium oxide mirror, as claimed in claim 4, wherein:said irregular reflection of said mirror is 1%; and said centerline average height is 0.3 nm.
 9. A titanium oxide mirror, as claimed in claim 5, wherein:said irregular reflection of said mirror is 1%; and said centerline average height is 0.3 nm.
 10. A titanium oxide mirror, as claimed in claim 7, wherein:said irregular reflection of said mirror is 1%; and said centerline average height is 0.3 nm. 